The present invention relates to an image forming apparatus having an electro-photographic or electrostatic recording type fixing device.
Normally, on a fixing device used for an electro-photographic image forming apparatus, a fixing nip is formed, for example, by fixing roller and pressing roller and a transfer material carrying toner thereon is fixed on this fixing nip.
Conventionally, during this fixing process, offset of toner in which toner sticks onto the fixing roller surface is caused or winding of paper in which a sheet carrying toner thereon is rolled up around the fixing roller is caused. Although the above has resulted in a problem that the image quality and paper feeding performance are deteriorated, a technique of applying, for example, silicone oil on the fixing roller surface has been employed for preventing the offset and paper winding. In the Japanese Patent Application Laid-Open Publication No. 11-7216, for example, a fixing device using oil is disclosed.
With a conventional technique of using silicone oil on the fixing roller, however, silicone oil on the fixing roller adheres onto the transfer sheet and so sticky touch due to the silicone oil adhesion cannot be avoided and the quality of the image produced is questionable. In addition, silicone oil is expensive and also it becomes necessary to install a mechanism for supplying silicone oil to the fixing device on the apparatus. Moreover, part of silicone oil evaporates due to heat and may contaminate inside of the apparatus.
In view of the above, a technique that, by reducing the conventional use of silicone oil on the fixing roller as much as possible and instead by using wax-added toner, wax melted out from the toner by heat is supplied to the fixing roller and that the offset and paper winding is prevented is disclosed in the Japanese Patent Application Laid-Open Publication No. 2001-125414 and HEI 3-185459 (1991).
Use of wax-added toner as above enables to reduce the use of silicone oil and prevent the offset and paper winding.
When wax-added toner is used, however, part of wax infiltrates out from the toner onto the fixing roller adheres to the fixing roller and a latent image caused by wax is formed on the fixing roller. If the next fixing step is carried out under the above condition, the wax forming a latent image is fixed on the image surface thereafter. It is found that, as a result of the above, wax is distributed abnormally on the image surface and that a phenomenon of uneven gloss or uneven transmission in the case of OHT (hereinafter called gloss memory) is caused. The gloss memory is very remarkable on a uniform plain image and degrades the image quality as it generates uneven gloss or uneven transmission due to the wax latent image on transfer sheet or OHT. This becomes particularly remarkable on a color image of which the above plain image occupies wider area, resulting in deteriorated image quality.
The afore-mentioned Japanese Patent Application 11-7216 describes a procedure of toner cleaning for removing the residual toner on the fixing roller, using an external heating roller and web, but does not mention about toner including wax nor about the above phenomenon caused by the wax. This document contains a description that specifies the surface roughness of the external heating roll (average surface roughness of not more than 0.07 mm), but an image forming apparatus like this cannot resolve the phenomenon involved in an image forming apparatus that uses toner including wax.